Helical piles, also known as screw piles or screw anchors, are structural deep foundation elements used to resist against forces exerted by axial compression, tension, and/or lateral loading (Bradka, 1997). Typical helical piles utilize one or more helical plates affixed around one end, the toe end, of a continuous central shaft of smaller diameter with a connection plate at the opposite or top end. Multiple helices used on the toe end of a central shaft can be of equal diameters or have a smaller diameter towards the toe of the pile. Helical piles are usually, but not exclusively, fabricated from steel that can also be galvanized for extra protection against corrosion. Helices are attached to the shaft most commonly by welding, but may also be bolted, riveted, or monolithically made with the shaft (Bradka, 1997).
In use, a helical pile is basically rotated into the soil such that the helical plate engages with the soil to advance the pile into the ground. As a result, there is minimal or no vibration associated with the installation of helical piles, unlike most driven piles. Further, the helices are configured for soil displacement rather than soil excavation, so there is little or no spoil to be removed. Once the helical pile has been installed to depth the helical plates create a bearing surface to distribute the axial load to the surrounding soil.
Helical pile capacity is based on soil parameters and equations relating soil parameters, size of helix(s), shaft size and length, to capacity. Soil, however, is rarely homogenous and is highly variable so soil parameters used in design, along with capacity, must be validated in the field to ensure the foundation will perform as designed. Typically soil parameters are determined through geotechnical bore holes drilled at various locations and represent only a sample of what may be encountered due to the high variability. The capacity of deep foundations is typically validated through load tests which are costly, time consuming, and cannot validate a 100% of the installed piles. Accordingly, a large margin for error in actual capacities vs. calculated capacities can exist when calculated capacities are based on geotechnical bore hole and load test alone. Therefore, it is industry practice to use a measure of the force, in the form of torque, required to drive the helical pile into the ground as a way to verify that soil parameters used in design are similar at each pile location to what was used for the design. Through the collection of large amounts of load testing data, along with collecting the torques applied during installation, and correlating the load testing data and the torques applied during installation, the amount of torque required to achieve a certain load carrying capacity can be determined, and the soil parameters used in the design can be validated to show the soil parameters used to design the pile and/or the pile configurations are in fact valid at each pile location. Unfortunately, the mechanisms and techniques for measuring the torque during installation of a helical pile typically involves measuring the hydraulic fluid pressure required to rotate the drive head motor as the helical pile is being installed. The theoretical pressure conversions to torque are derived from the manufacturer of the drive head motors, and the hydraulic fluid pressures are then used to calculate the estimated torque that is applied to the helical pile as it is being rotated into the ground. The accuracy of this method of measuring torque is highly variable and depends on many factors, such as motor efficiency, length and configuration of hydraulic hosing, hydraulic back pressures, speed of installation, and downward or upward force being applied. In particular, the amount of downward or upward force being applied to the pile during installation can give way to false torque readings and is highly influenced by different operator's methods. This method of using hydraulic fluid pressure to estimate torque is further complicated by the difficulty of recalibrating the pressure to torque correlation factor of the drive head as the motor wears and loses efficiency.